1. Field of the Invention
The present invention relates to a process for the catalytic conversion of methanol to formaldehyde. In particular, the present invention relates to a process for preparing formaldehyde from methanol using an adiabatic, silver catalyzed primary reactor and an adiabatic, metal oxide catalyzed secondary reactor wherein the air used in the reaction is also used to strip methanol and water from the product.
2. Prior Art
There are two commercially accepted processes for converting methanol to formaldehyde. The first utilizes a silver catalyst and operates in an oxygen lean atmosphere. The second utilizes a metal oxide catalyst and operates in a methanol lean atmosphere. The first process involves passing a mixture of methanol vapor and air over a fixed bed catalyst at approximately atmospheric pressure and absorbing the product gases in water. The mechanism is believed to be a combination of two reactions involving the dehydrogenation and oxidation of methanol: EQU CH.sub.3 OH.fwdarw.HCHO+H.sub.2 EQU CH.sub.3 OH+1/2O.sub.2 .fwdarw.HCHO+H.sub.2 O
Silver-catalyzed processes for making formaldehyde from methanol can be characterized according to the number of catalytic stages used to effect the conversion. Single stage operation is quite widely used but suffers from the disadvantage that rather high amounts of unconverted methanol are contained in the product emerging from the catalyst bed. This phenomenon is customarily referred to as methanol leakage. Since for many applications methanol is an undesirable contaminant, it must be separated from the formaldehyde solution. This entails a substantial investment in distillation facilities and energy to carry out such separations. It is usually necessary that the methanol content of the product be no greater than 2% by weight.
One way of eliminating the need for facilities to distill off methanol is to use two catalytic stages with interstage cooling. A basic two-stage process for this type is disclosed in Meath U.S. Pat. No. 2,462,413. In Northeimer U.S. Pat. No. 3,959,383 in improvement on the Meath process is disclosed by which even lower amounts of methanol in the product can be obtained.
In addition to the above-described dual silver two-stage processes, other two-stage processes have been described in the art. U.S. Pat. No. 2,519,788 to Payne describes a process for converting methanol to formaldehyde using an adiabatic first stage reactor containing a silver catalyst and an isothermal second stage reactor containing a metal oxide catalyst.
U.S. Pat. No. 4,119,673 to Aicher et al discloses a process utilizing a single stage conversion of methanol to formaldehyde over silver catalyst wherein the air used in the reaction enters the reaction system through a stripping column.